Publish our type annotations
As defined by PEP 561 (https://www.python.org/dev/peps/pep-0561), add a
py.typed
file to the BuildStream package. This allows any downstream
packages (currently plugins, maybe other use cases in future) to run
type checkers against BuildStream as well.
Although we don't have type hints for the private API surface, downstream packages should only be using public API anyway, so they should mostly be fine.